How to check if joists are on plane?

Question

I'm considering using 1x3 strapping before applying my new drywall ceiling (after doing some more work on them the other night and realizing it may be the best option). The ceiling joists are exposed and the walls at the top have a gap from about 1/2-3/4" from using a utility knife during tear down. I'm not overly concerned with gap in excess of 1/2" because taping and mudding will hide it, plus I'm adding crown with backing at the end.

What I want to do before installing the strapping is to ensure the joists are in the same plane with one another. I don't have a laser level, but I do have a 4' bubble level and some string. My thinking was I could run some string at various intervals across the joists, having it flat against the first and last joists and checking that it's tight against all the other joists. I could then confirm by measuring from the joists that cause the gaps (meaning they are the lowest) to the floor.

Any suggests on another way (without laser level) to achieve these?

Answer 1

Your plan for using builder's string is good, but don't set the string tight. That opens you up to cumulative contact error. You'll end up with a soup bowl for a ceiling.

Instead, put a spacer of a convenient thickness under the string at the outside. I've often used a scrap of 1x or 2x lumber (3/4" and 1-1/2", respectively). Then you'd measure the gap at intervals, either by setting a series of lines across the room, or by swinging a single line from a corner across to various points along the opposing wall.

If you find high or low spots, here are a couple remedies...

• For low spots, sister blocks or strips to the joist as needed, or use shims under your strapping. You can still screw your strapping to the joist, so these don't need to be thick enough for screws. They just act as a spacer.

• For high spots, chalk a line from end to end on one side of the joist, anchoring at the bottom corner. Use a circular saw or power planer to remove the hump (sag).

Note that level isn't really what's important here. No one can see a mild slope in a ceiling. Flat is your priority.

Answer 2

You're assuming that your floor is level; this will lead to issues.

What you can do is mark several level spots on the wall which runs parallel with your joists. Since you have a 4' level I recommend putting it on the wall and marking a dot every 2 feet or less so that when you go to mark your next dot you can line up the level on 2 previous dots instead of having to meticulously keep track of the bubble.

Attach string at each spot you marked.

Now run the string to the other wall (the string should be running perpendicular to the joists), use a string line level, and attach the string to the other wall.

This will let you see and measure whether your joists are all level in relation to your string.

Answer 3

The approach by @isherwood is correct. However, a very simple tool I've used that may make it simpler to fine-tune the plane is an 8 foot aluminum straight edge. They're inexpensive, about $25. The one I use is anodized aluminum (light), is easily broken down into four foot sections and has virtually no flex, especially if used on edge.
By sliding it across the surface of the joists you can quickly see where high spots and low points occur. The best approach IMO is to plane down the highest spots first and then build up low areas with blocking or shims as needed.
Edit: Since this is a ceiling using a straight edge to identify irregularities is easier done with two people. One working the straight edge and the other marking the high or low points on the joists.